An electrophotographic printer forms an electrostatic latent image on a photosensitive drum by light according to an image signal, develops the latent image by selectively attracting toner thereto, and then transfers the developed image onto a paper to obtain a print image. As a light source for forming the electrostatic latent image, a laser and a light-emitting diode array are widely used. In particular, since a light source constituted by the light-emitting diode array does not need a long optical path unlike the laser-type light source, it is suitable for small-sized printers and large-sized printing. Demand has been mounting recently on light-emitting diode arrays of higher precision, higher emission power and low in cost, as printing has been becoming faster with higher image quality, and as printers have been becoming smaller.
Usable to produce low-cost light-emitting diode arrays are not the provision of ICs for separately driving light-emitting diodes (static driving system), but the arrangement of light-emitting diodes in the number of the n-th power of 2 in one block, which are subjected to time division operation by a switching matrix wiring, thereby reducing the number of driving ICs and bonding wires (dynamic driving system).
In the production of such light-emitting diode arrays, Ti/Au and Mo/Au are widely used as wiring materials. The formation of these wirings may be conducted by a lift-off method, which comprises photoresist patterning, electron beam-heating vapor deposition and the removal of unnecessary portions together with the photoresist, and a sputtering/ion-milling method, which comprises depositing Ti/Au, Mo/Au, etc. onto the entire wafer by sputtering, and physically removing a patterned photoresist and an insulating mask of SiO2, etc. Because the lift-off method has insufficient step coverage, wiring layers in LEDs having large mesa-etched steps are easily broken at the steps. Though wiring layers formed by the sputtering/ion-milling method have good step coverage and are resistant to breakage at the steps, Au disadvantageously tends to remain at the mesa-etched steps. Particularly, LEDs of the dynamic driving system comprising common electrodes are disadvantageous in the short-circuiting of wirings by Au remaining after working, resulting in undesired light emission.
FIG. 4 is a plan view showing a conventional light-emitting diode array of a two-divided dynamic driving system. FIG. 5(a) is a plan view showing one of blocks shown in FIG. 4. FIG. 5(b) is a cross-sectional view taken along the line A-A in FIG. 5(a), and FIG. 5(c) is a cross-sectional view taken along the line B-B in FIG. 5(a). A plurality of light-emitting portions 1 is arranged at predetermined intervals on a p-type GaAs conductive layer 11 formed on an n-type GaAs substrate 10. Each light-emitting portion 1 comprises a p-type AlGaAs etching stopper layer 12, a p-type AlGaAs clad layer 13, a p-type AlGaAs active layer 14, an n-type AlGaAs clad layer 15, and an n-type GaAs cap layer 16, which are laminated on the p-type GaAs conductive layer 11 in this order. The light-emitting area of the light-emitting portion 1 has a double-hetero structure composed of the p-type AlGaAs clad layer 13, the p-type AlGaAs active layer 14, and the n-type AlGaAs clad layer 15.
Each light-emitting portion 1 is formed by removing part of epitaxial layers by mesa etching. The mesa-etched groove comprises a first mesa-etched groove 19 separating the light-emitting portions 1 from bonding portions 8a, 8c, and second mesa-etched grooves 20 dividing blocks from each other.
Each light-emitting portion 1 has a cathode 2 formed on part of the upper surface thereof. An anode 3 provided on each p-type GaAs conductive layer 11 adjacent to the light-emitting portion 1 is formed by vapor-depositing metals on the p-type GaAs conductive layer 11 and turning the metals to an alloy. Except for contact holes 7c, 7a for the cathode 2 and the anode 3, the light-emitting portions 1 and an exposed surface of the conductive layer 11 are covered by a first insulating film 17 made of PSG (phosphorus glass). Common electrodes 4, 4 are formed on the first insulating film 17 covering the bonding portion (mesa-top surface) 8c. After covering the common electrodes 4, 4 with a second insulating film 18, an Au wiring layer 5c is formed, such that it extends from one end connected to the cathode 2 not covered by the second insulating film 18, to the other end formed on a bonding portion 8c, which is connected to one of the common electrodes 4, 4 and constitutes a bonding pad 6c. An Au wiring layer 5a is formed, such that it extends from one end connected to the anode 3 to the other end on the surface of the bonding portion (mesa-top surface) 8a, which constitutes a bonding pad 6a. 
When the Au wiring layers 5 are formed on the light-emitting diode array of such a structure by a sputtering/ion-milling method, Au tends to remain on the slanting surfaces 21 of the first mesa-etched groove 19 after ion-milling, unsatisfactory to prevent the short-circuiting of Au wirings (see JP 2000-323750 A). Though this problem can be solved by removing all epitaxial layers by the first mesa-etched groove 19 except for the light-emitting portions 1, and forming wiring layers, common electrodes and bonding pads on the bottom surface of the mesa-etched groove, another problem occurs. That is, when all epitaxial layers are etched except for the light-emitting portions 1, there is difference in depth between large etching areas and small etching areas between the light-emitting portions. Because an etching speed depends on an area to be etched (loading effect), it is extremely difficult to control the dimension the light-emitting portions affecting the performance of the light-emitting diode array.
JP 10-157193 A discloses a light-emitting diode array having recesses in part of a semiconductor substrate, in which common electrodes are formed. However, because only common electrodes are formed in the recesses in this system, Au remaining on the slanting surfaces of the mesa-etched grooves tends to cause short-circuiting.